1. Field of the Invention
The present invention relates to a method of manufacturing a sensor and a sensor.
2. Description of the Related Art
Conventionally known sensors include oxygen sensors configured such that a detection element assuming a tubular shape (hereinafter also referred to as a tubular detection element) is housed in a metallic shell. Some oxygen sensors employ a tubular detection element having a flange portion, which is integrally formed at a substantially central position in the axial direction such that the flange portion faces rearward and projects radially outward. The tubular detection element is disposed in the metallic shell such that the flange portion is engaged directly or via another member (such as a packing) with a stepped portion projecting radially inward from the wall surface of a through-hole of the metallic shell.
Meanwhile, recently proposed oxygen sensors employ a detection element having a plate shape (a detection element having a laminated structure; hereinafter also referred to as a plate-type detection element) in place of a detection element having a tubular shape. Such a plate-type detection element does not have an integrally formed flange portion for engaging a stepped portion of a metallic shell. Thus, the plate-type detection element employs a tubular insulator on which an engagement surface is formed for engaging the stepped portion of the metallic shell, so that the plate-type detection element can be held by the metallic shell. More specifically, the insulator and the plate-type detection element are fixedly united, and the engagement surface of the insulator serves as the engagement surface of the flange portion and is engaged with the stepped portion of the metallic shell, whereby the plate-type detection element is held by the metallic shell (refer to Patent Document 1).
Use of the insulator as a flange portion enables attachment of a plate-type detection element to a metallic shell designed for use with a tubular detection element. Thus, sensors employing a tubular detection element and sensors using a plate-type detection element can share metallic shells as common components. Such sharing of components reduces component costs in the production of sensors.
Meanwhile, some oxygen sensors that employ a detection element having a plate shape are configured such that an insulating holder, a powder-compacted filler, and an insulating sleeve are arranged in layers so as to surround the plate-type detection element from all radial directions. In the process of manufacturing such a sensor, after the insulating holder and the powder-compacted filler are disposed within a metallic shell, a plate-type detection element to which the insulating sleeve is attached is inserted through an element-insertion through-hole of the powder-compacted filler and through an element-insertion through-hole of the insulating holder, whereby the plate-type detection element and the metallic shell are joined together. Subsequently, pressure is applied to the powder-compacted filler. Compression stress of the powder-compacted filler causes the plate-type detection element to be held in the metallic shell, thereby uniting the plate-type detection element and the metallic shell (refer to Patent Document 2).
[Patent Document 1]
Japanese Patent Application Laid-Open (kokai) No. 2002-174622 (FIGS. 1 and 3)
[Patent Document 2]
Japanese Patent Application Laid-Open (kokai) No. 2002-168823 (FIG. 1)
3. Problems to be Solved by the Invention:
However, the conventional oxygen sensor disclosed in above-mentioned Patent Document 1 employs a glass seal material to fixedly join the insulator and the detection element. Glass welding requires high-temperature thermal processing. Since high-temperature thermal processing may crack (fracture) the detection element with an abrupt change in temperature, thermal processing time must be set long with a slow rate of raising the temperature so as to prevent fracturing of the detection element. However, in the process of manufacturing a sensor, increasing the high-temperature thermal processing time reduces manufacturing efficiency.
Meanwhile, the conventional oxygen sensor disclosed in above-mentioned Patent Document 2 employs a powder-compacted filler (a ring member formed through compaction of a powder substance (hereinafter also referred to as a powder-compacted ring)) in place of the glass seal material, thereby eliminating the need of a thermal processing step for glass welding and thus avoiding reduced sensor manufacturing efficiency, which could otherwise result from increasing the high-temperature thermal processing time.
The powder-compacted ring can be formed into, for example, an annular shape having an element-insertion through-hole formed at a central portion. The element-insertion through-hole has a cross-sectional area slightly greater than that of the plate-type detection element so as to allow insertion of the plate-type detection element therethrough.
However, in manufacturing a sensor that uses a powder-compacted ring having an element-insertion through-hole formed in the above-mentioned shape, the following assembly work becomes troublesome: after the powder-compacted ring is disposed in a through-hole of a metallic shell, a plate-type detection element and the metal shell are assembled together such that the detection element is inserted into the element-insertion through-hole of the powder-compacted ring. Specifically, when the plate-type detection element is to be inserted into the element-insertion through-hole, the plate-type detection element must be set to an angular orientation (an angular position around its axis) so as to be insertable into the element-insertion through-hole. Particularly, in the case of a metallic shell configured such that the powder-compacted ring is disposed deep in its through-hole, identifying the angular orientation of the element-insertion through-hole of the powder-compacted ring becomes difficult; as a result, assembling the plate-type detection element and the metallic shell becomes troublesome.